1. Field of the Invention
The invention relates to a method and to a device for testing an exhaust gas sensor and, in particular, an NOx sensor for motor vehicles with a diesel internal combustion engine.
2. Description of the Prior Art
Due to the increasingly strict legal requirements in terms of permissible emissions of pollutants in motor vehicles in which internal combustion engines are arranged there is a need to keep the emissions of pollutants during operation of the internal combustion engine as low as possible. This can be achieved, for example, by reducing the emissions of pollutants which are produced during the combustion of the air/fuel mixture in the respective cylinder of the internal combustion engine. On the other hand, exhaust gas post-treatment systems are used in internal combustion engines, said exhaust gas post-treatment systems converting the emissions of pollutants which are produced during the combustion process of the air/fuel mixture in the respective cylinder into harmless substances. For this purpose, catalytic converters are used which convert carbon monoxide, hydrocarbons and nitrogen oxides into harmless substances. Both the aimed-at influencing of the production of the emissions of pollutants during the combustion and the conversion of the components of the pollutants with a high degree of efficiency by means of an exhaust gas catalytic converter require a very precisely set air/fuel ratio in the respective cylinder. NOx sensors are used to determine the nitrogen oxide content in the exhaust gas. In this context it is necessary to ensure that the components of the exhaust gas post-treatment system also function in the desired way over a long service life and faults are reliably detected.
The manual “Handbuch Verbrennungsmotor” [Internal Combustion Engine Manual], published by Richard van Basshuysen/Fred Schäfer, 2nd Edition, June 2002, Friedrich Vieweg & Sohn Verlagsgesellschaft mbH Braunschweig/Wiesbaden, pages 589 to 590, discloses an NOx sensor based on a ZrO2 ceramic, which NOx sensor has two chambers. In the first chamber, a constant partial pressure of the oxygen contained in the exhaust gas is brought about by applying a pump current. The pump current is inverted proportional to the air/fuel ratio. In the second chamber, the nitrogen oxide contained in the exhaust gas is decomposed by applying a further current. This current is proportional to the nitrogen oxide content in the exhaust gas and forms the measuring signal of the NOx sensor.
Document DE 199 07 947 B4 discloses a circuit for an NOx measuring pickup which has a first measuring cell and a second measuring cell which is connected to the first measuring cell. The measuring cells are located in a solid electrolyte. The circuit has a first circuit arrangement which sets a different oxygen concentration in the first measuring cell than in the gas to be measured by tapping a first Nernst voltage which serves as a first reference variable. A second circuit arrangement sets a different oxygen concentration in the second measuring cell than in the first measuring cell by tapping a second Nernst voltage which serves as a second reference variable. A third circuit arrangement drives a pump current composed of oxygen ions which originate from NOx out of the second measuring cell by tapping a third Nernst voltage which serves as a third reference variable.
The exhaust gas sensor may be contaminated due to components in the exhaust gas. This may make it necessary to carry out diagnostics of the exhaust gas sensor during the ongoing operation of the internal combustion engine.